In mice, the germinal center (GC) supports post rearrangement diversification of Ig by V(D)J hypermutation and is necessary for the development of B memory cells. These processes require costimulus-dependent interactions between antigen-specific T and B lymphocytes and (factors from question mark) Follicullar dendritic cells (FDC). GCs also appear to serve as a peripheral site for the maintenance of self-tolerance by selective apoptosis. Indeed, in many ways this unique peripheral microenvironment recapitulates certain events of primary lymphogenesis; recently we have extended this recapitulation by the demonstration of abundant Rag-1, Rag-2, and TdT expression in GC centrocytes. Thus we propose to study the biological significance Rag and TdT expression in GCs, the role of mIg affinity in divine B cell activation proliferation, and selection, the structure of positively- and negatively selected anti-hapten antibodies, chromosomal fragmentation in GCs and the possibility of transforming translocations, and role of the CD21/CD19/TAPA-1 complex in selecting the memory B cell compartment.